The present invention is generally related to cartridges having at least two stratified propellant charges and more particularly to a cartridge having a hermetic barrier or seal between these charges.
There have been many efforts over the years to reduce peak chamber pressures and improve ballistic performance of cartridges by providing stratified propellant charges in the cartridge case. Providing a spacer wad or diaphragm to maintain separation of the charges in these cases is also well known. Ammunition cartridges for small arms have been known to have perforated barriers between the propellant charges since the 1800's. These barriers helped to delay the ignition sequence and the pressure rise within the weapon chamber and therefore reduced recoil.
One such early cartridge is disclosed in U.S. Pat. No. 34,615 to A. Shannon in 1862. This cartridge had three black powder charges and two perforated diaphragms separating the charges from one another. The perforations permitted flame propagation between the charges and the size of the perforations determined the speed of such propagation from one charge to the other.
Another early cartridge, disclosed in U.S. Pat. No. 751,519, disclosed a metal, paper or felt diaphragm with a perforation therethrough to facilitate flame propagation and sequential charge ignition. The sequential charge ignition was designed to increase the velocity of the projectile without increasing the pressure in the gun.
An ordnance cartridge having separated charges is described in U.S. Pat. No. 1,920,075. This cartridge has a plurality of charges each separated from the other by an elastic lacquer spherical layer or a salt disk with a central bore therethrough through which the priming flame is directed to the forward most charge so as to ignite the forward charge first.
More recently, U.S. Pat. No. 2,072,671 issued to W. B. Foulke, disclosed a cartridge having a first loose propellent in the case and a second propellant encapsulated within cellulose capsules randomly positioned within the loose powder charge.
Another recent U.S. Pat. No. 4,593,622 to Fibranz, discloses a cartridge having a gas permeable barrier of a material such as felt or fabric between two propellants having different burn rates and a tube through the base charge and the barrier to cause the primer to ignite the top charge first. The gas permeable barrier separates the charges while permitting propagation of the flame front between the charges.
Each of the designs in these patents requires a direct communication path between the charges for the sequential ignition of the charges to properly occur. This direct communication between the charges is not believed to be necessary, and in fact may reduce the ignition delay and the pressure reducing effect of the sequential charge ignition.
A study of several non-communicative barriers was conducted and summarized for the US Army in Report No. R-1885 entitled "Sequential Ignition of Small Arms Propellants by Means of Scheduling Coatings", by Ludwig Stiefel. Various barriers were tested such as nitrocellulose sheet, cellulose acetate sheet, Saran film, rubber sheet, etc. These barriers were disks positioned in the case after the first propellant charge was installed. In addition, a bag made of a rubber glove finger was used to contain the second charge. These trials indicated, as did the above patents, that the peak pressure could be reduced.
However, all of these patents and the report just discussed, do not suggest utilizing a hermetic barrier between the charges to delay the ignition by maintaining charge separation until a preselected pressure is reached. We have found that a hermetic barrier placed between the charges is particularly advantageous in that it both delays and reduces the peak pressure experienced during propellant ignition or burning.
The present invention is particularly directed to cartridges having a reduced neck diameter such that a full diameter solid barrier would be difficult if not impractical to install. The cartridge of present invention comprises a tubular case having a conventional head end and a reduced diameter open mouth end, at least two propellant charges contained within said case, and a hermetic barrier means installed for separating the charges from one another and preventing premature crossflow of ignition gasses.
One preferred embodiment of the barrier means comprises a formed in place layer of polymeric resin material which conforms to the surface of a first charge loaded into a cartridge case. The self-leveling liquid resin must be viscous enough to stay primarily on the surface of the charge during the time it takes for the resin to cure into a solid layer.
An alternative is to use a very viscous fluid resin and then forcefully spread it over the charge surface.
If a less viscous liquid resin is used, a support disk must be used to prevent the resin from seeping into the propellant column. The layer may be formed by first installing a disk of kevlar fabric (could also use glass and other fabrics/fibers) or nitrocellulose against the first charge and then applying the liquid resin so as to coat the surface and edges of the disk to form the seal between the case wall and the charges. The resin must be fairly quick curing to form an impermeable barrier over the first charge without seeping through the first charge and prior to loading the next charge.
Another form of barrier in accordance with the invention is a flexible, elastically collapsible disk made of polyethylene or other flexible plastic material. The disk may be dish shaped or preferably may take the form of an umbrella which has a plurality of radial pleats to facilitate the collapse of the disk into a generally conical shape for insertion through the narrow case mouth. The elastically collapsed disk is inserted with the apex pointed toward the case mouth, i.e. the base end first. When the collapsed disk is fully inserted past the neck, it spreads to the inside diameter of the case. At this stage of insertion, the disk is still convex toward the mouth end. Upon seating the disk against the charge, the center of the disk is flexed and inverted so as to present a concave surface toward the case mouth and a convex surface toward the head end. The installed disk is thus placed in compression against the case to provide a flexible, resilient seal biased against the case to maintain the charge in position.
The disk may also have a tubular rim to provide a greater contact surface against the case wall when in the installed position. This rim presents a flat surface against the wall. The rim also can permit a tighter seal than simply the disk itself.
Another envisioned alternative preferred embodiment of the invention takes the form of a disk made of a shape memory material. The disk is formed into a curved shape below its transition temperature allowing it to be inserted through the core mouth. Following insertion, when the disk is heated above its transition temperature, it recovers to the flat disk shape. In this embodiment, the disk is slightly larger in diameter than the inner diameter of the cartridge case so that when the temperature exceeds the transition temperature, the disk will flatten out, pushing against the case wall, placing it in compression against the case wall.